Hair Particle System to Create Forest: Some Trees are Floating
If the surface that the particles are emitted from is affected by another modifier then you will need to make sure that the particle system's modifier is below the other modifiers. This is so that the particle system can 'see' the previous modifier's result (because blender's modifiers execute in the order they are in the modifier 'stack', with each modifier passing on its data to the next).

Another option you may need is the Use Modifier Stack, found in the particle system's Emission panel. This makes sure the particle system is seeing the 'final result' of all the modifiers being applied to the object before trying to emit particles.